[acdha]

Those don't modify or cache the content, only serve it to the user which the publisher approved serving it to, and increasingly with HTTPS they can't even see it. Moreover, the publisher is implicitly accepting that by being on the Internet.

[posterboy]

Of course a lot of content is cached along the route, for frequent access urls.

[acdha]

Not by network hardware, which is the context of this discussion.

It's also not the type of activity being discussed, which is unauthorized caching: a content provider who uses a CDN is doing so intentionally and while shared local proxies are increasingly uncommon they also respect the Cache-Control headers set by the source — see e.g. https://redbot.org/?uri=https%3A%2F%2Fwww.wsj.com — so again there's the distinguishing factor of authorization.

[posterboy]

I'm absolutely foggy about the specifics, but I wasn't talking about CDN, rather about the ISP-side.